Types of Transmission

Types of Transmission

In general two types of system are used for the transmission

1. Overhead system

2. Underground system

1. Overhead system

In this system, the transmission of electrical power is by using overhead transmission lines over long distances. In such system, the appropriate spacing is provided between the conductors, at the supports as well as at the intermediate points. This spacing provides insulation which avoids an electric discharge to occur between the conductors. The transmission by overhead system is much cheaper than the underground system. The overhead transmission lines are subjected to the faults occurring due to lightening, short circuits, breakage of line etc. but overhead lines can be easily repaired compared to underground system. It is also true that though such faults are rare, if occurred it is very difficult to find exat point of fault as transmission lines are very long. In the overhead system, the insulation must be provided between the conductor and supporting structure.

Key Point Hence the maximum stress exists between conductor and earth.

2. Underground System

The cables are generally preferred in underground system. All the conductors must be insulated from each other in the underground system. As voltage level is high, insulation required is more. Hence due to insulation difficulties, the voltage level used in underground system is below 66 kV while the voltage level used in overhead transmission lines can be as high as 400 kV. The maintenance cost of the underground system is less compared to overhead system. In crowded areas, overhead system using bare conductors is not practicable where underground system using cables is preferred. The line surges are suppressed by using the cables hence cable must be used for the last part of the connection which can save transformers and generators from the damage due to line surges.

Key Point In the underground system, the maximum stress exists on the insulation between the conductors. 

3. Comparison of Underground and Overhead Systems

Review Question

1. Write the comparison between overhead and underground transmission system.